otio.algorithms.flatten_stack() only flattens one level

If you have Tracks or Stacks within a Track, `otio.algorithms.flatten_stack()` doesn't flatten all the way.

We might need a separate `otio.algorithms.flatten_track()` and a recursive `flatten()` that deals with arbitrary nesting.

Start at otio.algorithms.flatten_stack() and inspect its current tests and callers. Reproduce the nested Track/Stack case described in the issue, then define and verify the chosen recursive behavior; done means arbitrary nesting is flattened consistently and regression coverage captures it.

Written by the indexing model from the issue text.